Introduction: Evidence for the association between long-term exposure to ambient air pollution and the incidence of asthma or COPD in adults is limited. With few exceptions, existing studies have not used robust exposure assessments, large cohorts, or considered PM2.5 exposures. Methods: We estimated time-varying four-year average residential exposures to PM2.5, PM2.5–10, and PM10 using nationwide spatiotemporal models for participants of the all-female Nurses' Health Study (NHS). The NHS is a prospective cohort that began in 1976 with biennial follow-up. Incident cases of asthma and COPD (1992 through 2000) were ascertained through validated case definitions of a self-reported physician diagnosis and subsequent use of an asthma medication within the past 12 months or report of a diagnostic test at the time of COPD diagnosis. Cox proportional hazards models were adjusted for potential confounders, and stratified models were used to examine effect modification by smoking status. Results: There were 934 incident asthma cases and 324 incident COPD cases during follow-up among 106,534 NHS participants. The average age of participants throughout the follow-up period was 61.5 years. In fully adjusted models in the full cohort, the hazard ratios (HRs) for a 10-ug/m3 increase in four-year average PM10, PM2.5, or PM10-2.5 were 0.94 (95%CI: 0.84-1.06), 0.90 (95%CI: 0.73-1.12), and 0.93 (95%CI: 0.77-1.13) for asthma, and 0.91 (95%CI: 0.75-1.10), 0.93 (95%CI: 0.66-1.31), and 0.82 (95%CI: 0.60-1.13) for COPD, respectively. Models stratified by smoking status were generally consistent with overall results. Discussion: We found no evidence that long-term exposure to particulate matter increased the risk of incident asthma or COPD in this cohort of older adult women.